1. Field of the Invention
This invention relates generally to an interleaver for use in communication systems and, more particularly, to a weighted interleaving system for correlated channel coding.
2. Discussion of the Related Art
Communication systems are increasingly expected to be capacity efficient and heavy demands are currently required on many different types of digital communication channels. With many of these communication channels, a relatively large amount of bit errors may occur in a relatively short period of time within a sequence of transmitted bits. Errors occurring in this manner are generally referred to as burst errors, and thus, such communication channels are typically referred to as bursty or fading channels.
Traditional coding methods for communication systems operating through a bursty or fading channel often employ some form of interleaving in order to make the communication system more reliable. As is known in the art, interleaving attempts to spread the effect of burst errors in time such that the bit errors are decorrelated and separated from one another. This repositioning of error bits tends to separate the error bits so that they can be processed in conjunction with an encoding and decoding communication system. A convolutional or block decoder is able to tolerate up to some fraction of its input bits degraded or erased, known as the decoder's erasure threshold, and still provide acceptable performance, measured by bit error probability. The purpose of the conventional interleaver is thus to reduce the probability that the decoder's erasure threshold is exceeded.
Conventional uniform interleavers have an input-to-output delay distribution that is uniform in probability over an interval from 0 seconds to a maximum delay or length of the interleaver. When optimizing the performance of a conventional uniform interleaver, the only major trade involving the interleaver is its delay or length. If the interleaver is much longer than the correlation time of the channel, which is generally defined as the separation time at which fading probabilities become uncorrelated, the probability of a burst error at the output of the interleaver is small. The maximum delay or length of the interleaver is thus generally related to the fade correlation time of the channel. Although a uniform interleaver is not necessarily optimized for a particular channel, it is generally effective when the maximum delay or length is chosen to be much larger than the fade correlation time of the channel. As the fade correlation time becomes longer, it is generally not feasible to make a uniform interleaver of sufficient delay or length to provide adequate decorrelation of the faded information. With longer delays or interleaver length, a delay in data acquisition occurs. Such an information delay degrades the quality of real time signals especially in two-way voice communications. Consequently, long uniform interleavers are not well-suited to systems that require fast acquisition.
What is needed then is a weighted interleaving system for correlated channel coding which is chosen to minimize expected correlation between deinterleaver output bits without suffering from the disadvantages associated with conventional uniform interleavers. This will, in turn, increase data acquisition time; increase quality of real time signals; decrease the probability that the average erasure fraction exceeds the decoder's threshold; remove the restriction of a uniform interleaver delay; and match the length and shape of the interleaver to channel correlation statistics. It is, therefore, an object of the present invention to provide such a weighted interleaver system for correlated channel coding.